(1) Field of the Invention
The present invention relates to a cake of synthetic fibrid which is easy to handle and excellent in adaptability to the papermaking operations and which can be formed into a paper product having excellent properties.
(2) Description of the Related Art
Preparation of pulpy particles called "fibrid" from various synthetic polymers are known and described in the specifications of U.S. Pat. No. 2,988,782 and U.S. Pat. No. 2,999,788.
According to these known techniques, fibrids are prepared by dissolving a fiber-forming polymer such as an acrylonitrile type polymer, nylon, or polyethylene terephthalate in a solvent for the polymer to form a polymer solution (dope) and causing precipitation in the polymer solution under a strong shearing action in a precipitating agent which is a non-solvent for the polymer and has affinity with the solvent.
As the precipitating apparatus for preparing fibrids, a precipitating apparatus comprising a rotor and a stator in combination is disclosed in Japanese unexamined patent publication (Kokai) No. 52-15621 and
U.S. Pat. No. 3,018,091. The apparatus disclosed in Japanese unexamined patent publication No. 52-15621 is preferred because good fibrids can be prepared at a high efficiency.
In each of the known methods, a precipitated fibrid is dispersed in the precipitating agent to form a slurry. Since the fibrid per se has a good liquid-retaining property, even after the washing liquid is separated at the washing step, a large amount of the washing liquid is left in the interior of the fibrid and/or in the spaces among individual fibrids. Accordingly, even if the washing operation is carried out repeatedly, the washing effect is low and complete washing is almost never attained. Research has confirmed that even after hydroextraction (hereinafter, may be referred to as "dehydration"), water containing a large amount of a solvent is left in the fibrid aggregate in an amount 10 to 30 times the amount of the fibrid (as solids). Even if this fibrids is washed by using water in an amount 100 times of the amount of the fibrid according to the above-mentioned method, the amount of the residual solvent is merely reduced to 1/3 to 1/10 of the original amount.
The so-washed fibrid is hydroextracted by a vacuum filter, such as a Nutsche filter, to obtain a product. If hydroextraction is strictly carried out at this step, it is difficult to redisperse the fibrid in water at the subsequent step. Even if this fibrid is subjected to a papermaking operation, the touch and physical properties of the resulting paper-like product are poor and it is difficult to obtain a good paper-like product.
For example, a solution of a poly-m-phenyleneisophthalamide polymer in N-methyl-2-pyrrolidone (sometimes referred to as "NMP" hereinafter) is prepared. An aqueous solution of NMP is prepared as a precipitating agent. The two solutions are introduced into the apparatus disclosed in Japanese unexamined patent publication No. 52-15621 to precipitate the polymer. When the fibrid is washed with water, filtered, and compressed as a mass to various water contents, it is seen that if the average moisture in the pulp bale is smaller than 4 times the amount of the fibrid (the absolutely dry weight of the fibrid as solids), the dispersibility of the fibrid at the papermaking step is poor and the physical properties of the resulting paper-like product are inferior. Accordingly, the fibrid is practically hydroextracted to such an extent that the amount of water is about 4 to 5 times the amount of the fibrid. If the dehydration degree is further increased, a paper-like product having good properties cannot be obtained.
However, a fibrid aggregate containing such a large amount of water is difficult to handle and the transportation cost is extremely high. Moreover, there is a risk of deformation or breakage of the aggregate form during the transportation or there is risk of evaporation or leaking of water. Accordingly, it has been considered that fibrids are not suitable for long-distance transportation. This disadvantage has heretofore been obviated by conducting a fibrid-preparing process and a papermaking process in one factory or in adjoining factories.
Recently, use of a synthetic fibrid as a frictional member has been proposed (see U.S. Pat. No. 4,324,706) as means for solving the problem of environmental pollution by asbestos, and special molding has been adopted for preparing a paper-like insulating material from a synthetic fibrid. As uses and application methods of synthetic fibrids have thus been diversified, it often happens that a papermaking factory or other processing factory is not located in the same place as the location of a fibrid-preparing factory. Accordingly, development of a synthetic fibrid being capable of reducing the transportation cost, having a good handling property at the tim time of transportation and causing no trouble at a fibrid-processing step such as a papermaking step is eagerly desired.